TRS80 wrote:There are "new build" Z80 PCB's available from time to time through N8VEM but modern isn't quite the same even if it is better and more reliable.
Andrew do you know much about the CP/M modification that could be done on the System 80 in the day? The hardware mod details and the details of the disk format that was used? Do you have any docs?
On the System 80 site I have rough docs of a John Gilbert 64k upgrade
, but I'm not sure that's the same thing as a CP/M mod. Were the disks written in a format similar to the Kaypro for example? Or Model 4 CP/M ( Montezuma Micro)? Do you know?
Some time in the future it would be fun to Mod one of my System 80 for CP/M.
The difficulty with CP/M on the Model I,III and System 80 is the memory map of the machines. CP/M expects the low memory to be RAM and applications load starting at address 0x0100. On these machines this is ROM. Add to that the IO devices (screen, keyboard, printer, etc) mapped into the 0x37xx memory block and you have machines that will not run CP/M without modification.
Generally this modification is a board that fits into the Z80 socket and changes the address map. The Omikron Mapper is one such board for the Model 1. I don't think the John Gilbert modification does the remapping but I will look at it closer.
Once you have the hardware sorted then you need a boot rom that knows how to read the CP/M operating system from the disk. The Omikron Mapper had this on board and gave the option of CP/M or TRS-80 mode on boot.
I have Ian's newsletter DVD so will have a look and see if there are any CP/M mods documented there. Aside from the hardware, finding a copy of the boot disk will be important.
While the same applications will generally run on all CP/M machines the operating system itself won't without a BIOS configured for the specific hardware. This means finding a documented hardware modification is part of the job. You also need to find a boot disk with a BIOS compatible with the modifications. While most of CP/M is device independant the BIOS is specific to the hardware.
This "device independance" resulting from having device specific code in the BIOS seperate from the rest of the OS was one of the ground breaking concepts CP/M introduced and is why it was available across such a variety of machines.
The little Compuspec Z80 board I wrote about handles the ROM/RAM issue in a neat way. When it boots it copies the ROM (which starts at 0x000 like the TRS-80) into RAM starting at the same address and then disables the ROM. Grant Searle's 9 chip CP/M does something similar by disabling the ROM in the CP/M boot loader.